Writing instrument



FIG. 1 is a front elevational view of an ornamental design for a writing instrument according to the present invention;

FIG. 2 is a left side elevational view thereof, the right side elevational view being a mirror-image thereof;

FIG. 3 is a back elevational view thereof;

FIG. 4 is a bottom plan view thereof; and,

FIG. 5 is a top plan view thereof.

All portions shown in broken lines are environmental and illustrative only and form no part of the claimed design. 

The ornamental design for a writing instrument, as shown and described. 